Focused ion beam (FIB) systems are widely used in microscopic-scale manufacturing operations because of their ability to image, etch, mill, deposit, and analyze with great precision. Ion columns on FIB systems using gallium liquid metal ion sources (LMIS), for example, can provide five to seven nanometer lateral imaging resolution. Because of their versatility and precision, FIB systems have gained universal acceptance in the integrated circuit (IC) industry as necessary analytical tools for use in process development, failure analysis, and most recently, defect characterization.
The ion beam of a FIB system typically scans the surface of the integrated circuit in a raster pattern. This raster pattern is often used first to produce an image of the surface showing the top lines and elements of the circuit. The image is used together with circuit layout information to navigate the ion beam around the integrated circuit to locate a specific element or a feature of the circuit. Upon moving the raster pattern to the local area of the feature of interest, the ion beam current is increased to cut into the die and expose circuit features in buried layers. The FIB system can then alter the exposed circuit by cutting conductive traces to break electrical connections or depositing conductive material to provide new electrical connections. A gaseous material is often directed to the sample at the impact point of the ion beam, and the ions induce a chemical reaction that selectively increases the etch rate or deposits material, depending on the gaseous compound that is used.
Until recently, milling applications for FIB systems on metal interconnects of integrated circuits have been limited to the sputtering of polycrystalline aluminum or tungsten. Both materials can be milled by rastering a beam of gallium ions across the area of interest. To increase the etch rate, a gas containing a member of the halogen group (bromine, chlorine, or iodine) is often directed to the impact point of the ion beam to enhance etching. The beam is typically scanned across the area to be milled using digital electronics that step the beam from point to point. The distance between points is referred to as the pixel spacing. When milling without gas, the pixel spacing is typically less than the beam spot size, that is, each subsequent beam position overlaps the previous position to ensure a uniform cut and a smooth finish. This method is referred to as `Default Milling`. Milling methods have been well documented, for example, in U.S. Pat. No. 5,188,705 to Swanson, et. al. for "Method of Semiconductor Device Manufacture".
In recent years, semiconductor manufacturers have begun a migration toward the use of copper as a replacement for aluminum interconnects. As manufacturers strive to increase the speed at which chips work, the use of copper interconnects provides several advantages over aluminum. For example, copper has lower sheet resistance and exhibits both improved metal line and line/via/line electromigration reliability.
The halogens that are used to enhance focused ion beam etching of other metal interconnect materials do not significantly enhance the etching of copper, and no other chemical suitable for routine FIB milling of copper has been found. For example, the etch byproducts formed during the use of halogen compounds on copper at room temperature have low volatility and tend to leave detrimental deposits on the sample surface and via side walls. Thus, the milling of copper conductors is slow because there are no suitable etch-enhancing chemicals. Moreover, the milling of copper without chemicals has been found to produce non-uniform material removal, even when the ion beam is applied uniformly. The non-uniform removal causes the floor of the etched area to become non-planar, which in-turn can cause unacceptable milling of the material under the copper conductor; i.e., as milling continues to remove the remaining copper from one region under the ion beam, significant damage is incurred by milling the exposed underlayer in areas where the copper has already been removed.
This non-uniform milling is thought to be attributable to the formation of a resistant Cu region that forms during milling and propagates across the area exposed to the ion beam. This region appears to channel incoming ions into the crystal, thereby reducing the sputtering rate of copper by the incoming ions. The region appears black when imaged using the ion beam in secondary electron mode, apparently because the increased channeling of the incoming ions also reduces the ejection of secondary electrons near the surface.
The region is thought to be composed of a Cu--Ga alloy. Once formed, this area inhibits the surface-sputtering rate by a factor of between about 2 to about 4. Because this area forms over time during the milling procedure and because of channeling variations, the rastered region etches non-uniformly. Scanned areas less than one micron wide do not always exhibit the formation of black regions and appear to etch uniformly in many cases. In addition, the use of either Br.sub.2 or Cl.sub.2 gas during milling does not appear to mitigate the formation of the resistant area, and the search for additional room temperature enhancement gases has been, so far, unsuccessful.
An object of the invention, therefore, is to improve the uniformity of charged particle beam milling of copper and other materials.
Another object of the invention is to improve the rate of charged particle beam milling of copper and other materials.
Yet another object of the invention is to improve the rate and uniformity of focused ion beam milling of copper and other materials without the use of a toxic or corrosive gas.
Applicants have discovered that the etch rate of copper conductors is increased over default milling and the sample is etched more uniformly if the ion beam moves initially, not in a raster pattern of overlapping pixels, but in a pattern in which the sample is typically milled at a series of non-contiguous points. As the milling continues, the milling points move progressively closer together until, to complete the milling, a raster pattern of overlapping points is typically milled. For example, a first dose of ions may be applied at milling locations or pixels spaced 0.5 .mu.m apart in the x and y directions. Subsequent doses can be applied at pixel spacings of 0.4 .mu.m, 0.3 .mu.m, 0.25 .mu.m, and 0.1 .mu.m. A final milling at a 0.05 .mu.m pixel spacing producing a uniform finish to the floor of the milled area. In this example, six milling steps are described. Successful milling of copper has been achieved with as few as two and as many as ten milling steps, depending on specific copper characteristics such as thickness and grain structures. Because the milling within a rastered area is more uniform, the invention reduces damage to the material lying under the copper conductor. This damage is caused by continuous milling of an area from which copper has been removed while attempting to remove the remaining copper from an adjacent resistant area within the rastered box.
The inventive milling method inhibits the formation of the etch-resistant area, perhaps by reducing localized heating of the copper conductor. Milling non-contiguous locations may allow energy imparted by the impact of the ion beam at one location to dissipate through the specimen before the beam returns to that area, thereby preventing the accumulation of energy necessary to cause formation of the etch resistant region. Other methods of allowing energy from the beam to dissipate before the etch resistant area forms may also be effective in preventing the formation of the etch resistant regions and are within the scope of applicants' discovery and invention. For example, the accelerating voltage of the focused ion beam could be reduced from the typical range of 30 keV to 50 keV to a reduced range, such as between 1 keV and 20 keV, thereby imparting less into the target per ion. The propagation of the etch resistant area may also be reduced because the inventive method creates holes that isolate the copper into segments, and the etch resistant areas may be unable to propagate across these holes.
The inventive milling method not only prevents formation of the etch resistant area, it also increases the milling rate of the copper conductor in general. Because of the finite edge resolution of the ion beams, the sides of the milled holes are not exactly perpendicular to the surface. This increases the exposed surface area of the target and causes some of the impacting ions to strike part of the target surface at a non-normal angle. Both the increased surface area and the non-normal angle of incidence are thought to increase the etch rate.
Because the invention does not depend on a chemical precursor to increase the etch rate and contribute to etch uniformity, it is not limited to any particular type of sample material, although the benefit of the invention are particularly apparent in milling copper, which tends to form etch-resistant regions. By not requiring the use of etch-enhancing gases, which are typically corrosive or toxic, problems associated with the handling of such gases are avoided.